Gengar

Mutate onto Haunter {3}{b}{b} (Cast this card by
placing it on top of a Haunter you control.)

Gnawing Curse — Whenever an opponent targets a creature they control, it gets -2/-2 until the end of turn and you gain 2 life.

Poltergeist — As long as an opponent has 10 or
more life, Gengar has +5/+0 and Infect.